1. Field of the Invention
The present invention generally relates to a magnetic resonance imaging system comprising 12 channel phased array antennas for providing homogeneous magnetic field to thereby obtain a head anatomic image with a high resolution and high signal to noise ratio (SNR). Particularly, the present invention relates to the magnetic resonance imaging system comprising a transmit-only antenna and a multi channel phased array antenna, which are separated from each other.
2. Description of the Related Art
The magnetic resonance imaging under high magnetic field is known to provide higher SNR and contrast (due to T2 (spin-spin) relaxation) than that using a low magnetic field. For the reason, an extensive research has been conducted into the magnetic resonance imaging system that operates under a high magnetic field. One of the most important issues in such a system is that a radio frequency (RF) antenna having a high SNR, homogeneous magnetic field, and low specific absorption rate must be designed.
As an amplitude of magnetic field for magnetization of hydrogen nuclei increases, the SNR may be improved. However, if the amplitude of magnetic field becomes higher, the frequency used in an RF antenna should inevitably be increased, that is, the wavelength of field generated at the antenna should become shorter. A decrease in the wavelength (that is, an increase in frequency) in turn causes attenuation in a target subject for imaging (e.g., a man's head) due to high permittivity and conductivity of the target subject. Moreover, the problem associated with attenuation due to radiation loss also arises. Such problems are inherent in RF circuits having an operation frequency higher than 100 MHz.
Furthermore, when a radio frequency increases, the magnetic field becomes inhomogeneous, that is, the field in a man's head is distorted by phase shift. Such distortion is caused by an increase of the radio frequency and the high permittivity of a man's head. In the conventional system, which uses the magnetic field of 1 Tesla, the wavelength of the field in a man's head is about 1 m, which is much longer than the width of the head so that the phase shift may be ignored. Thus, the homogeneous magnetic field can be maintained. However, in the system that uses the magnetic field of 7 Tesla, the wavelength of the field in a man's head is about 12.5 m, which is about a half of the width of the head. Accordingly, the phase shift of the field in the head comes near 180 degree. Moreover, in the cylindrical RF antenna, which is used generally, a leg surrounding the antenna (the conductive part having the same direction as that of the main magnetic force) forms field within the antenna, and thereby the above phase shift causes a standing wave in the head so that the field in the antenna becomes convex. This convex shaped field is attributed to dielectric resonance. Due to the dielectric resonance phenomenon, the center of an image becomes bright and its peripheral part becomes dim, thus making it difficult to observe the image. That is, different image signals may possibly be obtained from the same subject, depending on the position where the image of the subject is acquired.
Another problem may also arise, which results from a penetration depth effect. The penetration depth effect means how deeply an electromagnetic wave penetrates into a subject. The penetration depth of the subject depends on the magnetic permeability, permittivity, and conductivity of the subject. When a subject with high conductivity is loaded into an antenna, the penetration depth becomes shorter. This causes a kind of damping that prevents the radio frequency from effectively contributing to form the image. That is, the homogeneous magnetic field cannot be formed.
As described above, in the high magnetic field, such as 7 Tesla, the conventional two-way (receive and transmit) antenna that has been used in the low magnetic field, such as 1 or 1.5 Tesla, cannot render an image with sufficient quality. Accordingly, the magnetic resonance imaging system, by which the above problem can be solved, is needed in the art.